This invention relates generally to an apparatus for recording a color image, and more particularly to a color electrophotographic apparatus utilizing an electrophotographic process.
There are known various methods of recording a color image in accordance with image information from a computer or the like. Among these methods, one utilizing electrophotography is effective in recording a color image of a high picture quality at relatively high speed. In the color image recording utilizing an electrophotographic method, yellow, magenta, cyan and black toner images are formed on a photosensitive member (recording member), and the thus formed toner images are combined together in a superimposed manner on paper or an intermediate transfer member, thereby forming a multicolor image. This electrophotographic method requires many imaging processes, and therefore has a disadvantage that the apparatus is complicated in construction, can not be easily handled, and tends to have a large overall size. Generally, four developing machines, when arranged around one photosensitive drum, are different in construction from one another, and therefore four kinds of developing machines are needed, thus complicating the construction of the apparatus, and besides the diameter of the photosensitive drum is increased. Therefore, there has been proposed a method in which a photosensitive drum having a relatively small diameter is used, and four developing machines are switched (that is, sequentially operated) by the use of a mechanism for sliding or rotating the developing machine. In this method, however, the mechanism for moving the developing machines is required, and therefore the apparatus is complicated in construction, and also can not be handled fully satisfactorily.
On the other hand, there has been proposed a method in which a photosensitive belt is used, thereby providing a flat portion on the photosensitive member, and a plurality of (usually, three or four) developing machines are arranged over this flat portion. For example, Japanese Patent Unexamined Publication No. 2-213884 discloses a color electrophotographic apparatus in which three developing machines are arranged over an upper surface of a photosensitive belt extended horizontally, and toner images formed on the photosensitive belt are transferred onto an intermediate transfer belt in an superimposed manner. Japanese Patent Unexamined Publication No. 5-503377 discloses another such example in which developing machines are arranged utilizing a flat portion of a photosensitive belt. In this construction, the developing machines are arranged below the photosensitive member. There is also disclosed a construction in which a photosensitive belt is extended in a vertical direction, and developing machines are arranged in a stacked manner on one side of the photosensitive member. In such a conventional construction, the developing machines are arranged on one side of the vertically-extended photosensitive member in the form of a belt.
In the above methods utilizing the photosensitive belt, it is somewhat difficult to handle the belt, but since the developing machines do not need to be moved extensively, a switching mechanism for them is simplified, and therefore with such a construction, a full-color laser beam printer of a desk-top size, which can be relatively easily formed into a small size, can be achieved.
However, even these methods still have the following problems. In these methods, the developing machines are arranged parallel to the photosensitive member. Therefore, the toner in the developing machine, located at an upstream side of the photosensitive member, is liable to be included or introduced into the developing machine located at a downstream side of the photosensitive belt.
In the method as disclosed in Japanese Patent Unexamined Publication No. 2-213884 in which a plurality of developing machines are arranged above a photosensitive belt, the developing machines are arranged above the upper surface of the photosensitive belt, and therefore there is encountered a problem that toners and a carrier are liable to be spilt on the photosensitive belt. In the construction disclosed in Japanese Patent Unexamined Publication No. 2-213884, the photosensitive member and the intermediate transfer member are both in the form of a belt, and therefore it is difficult to properly register the two belts with respect to each other because of expansion and contraction of the belts, offset of the belts, and so on, and therefore when the photosensitive member and the intermediate transfer member are rotated several times so as to form a color image, it is difficult to precisely form the images of the respective colors.
In the construction as disclosed in Japanese Patent Unexamined Publication No. 5-503377 in which a plurality of developing machines are arranged below a photosensitive belt, toner and carrier deposited on the photosensitive belt are liable to be introduced into the downstream-side developing machines which are not effecting a developing operation. And besides, since it is necessary to feed a developing agent in the developing machine upwardly against the gravity, the developing machines are complicated in construction. Particularly when the developing machines are of the two-component developing type requiring an agitating chamber in which the toner and the carrier are agitated, the toner must be circulated between the agitating chamber and a developing roll. Therefore, in the developing machine located either above or below the photosensitive member, the toner must be fed against the gravity, and in either case the developing machines are increased in size, and are complicated in construction.
On the other hand, in the construction as disclosed in Japanese Patent Unexamined Publication No. 4-268581 in which a photosensitive belt is extended in a vertical direction, and developing machines are arranged in a stacked manner on one side of the photosensitive member, there is a less flow of developing agents against the gravity as compared with the type of developing machines which effect a developing operation from the upper side or the lower side of the photosensitive member, and therefore generally, the developing machines can be easily formed into a small size. Even in this construction, however, there is encountered a problem that when an electrostatic latent image on the photosensitive member is to be developed, the toners, spilt from the elevated developing machines, are liable to be introduced into the low developing machines.